iPhone was performing badly because of an old battery. Once it was confirmed that Apple was slowing down iPhones because of the battery I felt safest ordering a battery from iFixit. I'd ordered cheaper batteries elsewhere, but had been burned before when they didn't have near the lifetime of the original. I knew iFixit would provide a good battery.
Repair went ok, except I didn't realize I also needed to order the adhesive strips. I didn't order the whole repair kit (and my order was before iFixit dropped their whole kit price to match Apple's.) I had a set of tools from a previous iFixit iPhone repair. I love their tutorias.
I ended up re-using part of the old adhesive strip and some dots of super glue. I would not advise my method as the adhesive strips guarantee the battery won't slide around and short. Between the old parts of adhesive strip and dots of super glue I think the battery will remain secure and be removable in a few years when it needs replacing. however, I will be very aware of any loose-item sounds coming from my iphone and replace the strips immediately.
Don't forget the adhesive strips.